Diabetes mellitus is a common disease which is usually classified into insulin-dependent and non-insulin dependent types. Both types may be managed by diet, in association with insulin in the first type and a variety of drugs in the second. However, while the changes in blood glucose can usually be managed reasonably satisfactorily by conscientious patients and doctors, this does not prevent long term damage to many tissues as a result of the disease. This damage may take many forms but the major types are damage to the eyes, (retinopathy), nerves (neuropathy), kidneys (nephropathy) and cardiovascular system. While some patients develop isolated damage to one system or another, in most patients two or more types of damage occur together suggesting that the underlying mechanism is similar.
There are many approaches to reducing or preventing these forms of damage, which are collectively known as the long term complications of diabetes. This patent specification concerns two of them.
One approach is based on essential fatty acid (EFA) metabolism being deranged with a block in the conversion of the main dietary EFA, linoleic acid, to its first metabolite, gamma-linolenic acid (GLA) the first of the `6-desaturated` n-6 EFAs in the sequence shown in Table I below (in this context it should be noted that the term `6-desaturated EFAs` means EFAs beyond the 6-desaturation step and does not necessarily mean that the EFA has a double bond in the 6-position):
TABLE 1 ______________________________________ 18:2 delta-9,12 (linoleic acid) .vertline. delta-6 desaturase .dwnarw. 18:3 delta-6,9,12 (gamma-linolenic acid) .vertline. elongation .dwnarw. 20:3 delta-8, 11,14 (dihomo-gamma-linolenic acid) .vertline. delta-5 desaturase .dwnarw. 20:4 delta-5,8,11,14 (arachidonic acid) .vertline. elongation .dwnarw. 22:4 delta-7,10,13,16 (adrenic acid) .vertline. delta-4 desaturase .dwnarw. 22:5 delta-4,7,10,13,16 ______________________________________
As a consequence the levels of metabolites of GLA, including dihomogamma linolenic acid (DGLA) and arachidonic acid (AA), are reduced. Since these metabolites are important components of the structure of the membranes of nerves and of cells in all tissues, and since they are the precursors and constituents of cellular signalling systems such as prostaglandins and diacylglycerols, deficits of these compounds are damaging. In particular there is a deficit of prostaglandin E.sub.1 derived from DGLA and of prostacyclin derived from arachidonic acid, both of which are vasodilating agents essential for maintaining blood flow to the microcirculation in many tissues.
As normal conversion of linoleic acid to GLA is impaired in diabetic complications, treatment with GLA and/or with its further metabolites DGLA and AA is beneficial. Prostacyclin levels have been shown to be reduced in the nerves in diabetic animals and to be restored by treatment with GLA.
Another approach is based on damage that results from over-production of the glucose metabolite, sorbitol, in the cells of the body. Glucose can be converted to sorbitol by the enzyme aldose reductase, but this step is very slow at normal concentrations of glucose and sorbitol levels are therefore usually very low. At high concentrations of glucose, in contrast, conversion is greater and high levels of sorbitol may occur. These levels may have a variety of effects on cells, causing disturbance to the osmolarity and disrupting various aspects of cellular metabolism, including the inositol cycle. These effects have been suggested to be among the causes of diabetic complications. If this is the case then blocking the enzyme using inhibitors, or more broadly giving a material reducing the sorbitol levels, is likely to be valuable to treat/prevent diabetic complications. Such inhibitors, aldose reductase inhibitors, have been developed by many different pharmaceutical companies.
Further, a recent report (Cunningham et al, J. Am. Coll. Nutrition 13 (No. 4) 344-350 (1994)) shows ascorbic acid to be as potent as any of the aldose reductase inhibitor drugs in inhibiting the enzyme.
In studying treatments, we have developed a model of diabetic complications using the slowing of nerve conduction which takes place in the rat made diabetic by the injection of streptozotocin, which damages the insulin secreting cells of the pancreas. This slow nerve conduction can be normalised by insulin treatment. It can also be normalised by treatment with gamma-linolenic acid in various forms or by treatment with a range of aldose reductase inhibitors.
We have constructed dose response curves showing the restoration of normal nerve conduction by both GLA and the enzyme inhibitors, the experimental details being described in a variety of publications such as N. E. Cameron, M. A. Corter and S. Robertson in "Diabetes", Vol.40, pp.532-39 (1991). Essentially rats are made diabetic with streptozotocin and maintained for 6 weeks without treatment with GLA or other fatty acid, then treated for 2 further weeks with the fatty acid, either with or without other materials under test. The effect of the agent is shown as the degree of restoration of conduction velocity as compared to the untreated diabetic group.
Present Work
We have now investigated interactions between the effects of GLA and the aldose reductase inhibitor ZD5522. In initial work,each active was given at a threshold dose which would be expected to produce an approximately 5% improvement in conduction velocity in the diabetic model. It would be expected that the two treatments given together might at best be additive, leading to an improvement of the order of 10%. Instead, in several experiments, depending on the precise doses chosen, the actual improvement was in the range of 40-80%, approximately 1.5-6 times greater than would be anticipated. This is a totally unexpected effect and is of very great therapeutic significance, promising greatly increased efficacy of treatment and at the same time a lower risk of side effects because of the ability to get therapeutic effects with lower drug doses than currently used. Similar results were obtained using a second aldose reductase inhibitor, WAY 121509.
Furthermore, when ascorbate is used instead of drugs such as aldose reductase inhibitors (ARIs), there is the major advantage of use solely of natural materials, well known to be tolerated in large doses, and which moreover can very readily be used in single-compound form as ascorbyl-GLA etc.